In traditional power amplification of radio frequency signals, it is necessary, in principle, to choose either between a high efficiency or high linearity. For instance, a class C-type amplifier provides high efficiency while the linearity is insufficient for broadband applications for instance, whereas a class A-type amplifier is relatively linear but has low efficiency.
When using one and the same amplifier for the simultaneous amplification of several information signals modulated on different carrier waves, or when using linear modulation formats, such as QAM (QAM=Quadrature Amplitude Modulation) for instance, there is required a power amplifier whose linearity is extremely high, since it is essential in such cases that all the phase and amplitude positions of ingoing signal components are maintained in the amplification process. Otherwise, intermodulation can occur between the signal components and/or the spectrum of the amplified summation signal can be broadened so as to manifest the risk of interference with signals transmitted via other channels. Amplifiers that are adapted for simultaneous power amplification of a plurality of narrow band channels, and power amplifiers that are intended to amplify a carrier wave on which several CDMA channels CDMA=(Code Division Multiple Access) are superimposed are examples of equipment on which the aforesaid high linearity requirement is placed.
D. P. Myer investigates how a high bandwidth linear power amplifier can be obtained in his article "A Multicarrier Feed-Forward Amplifier Design", Microwave Journal, October 1994, pp. 78-88. The forward feed of the input signal to the power amplifier enables certain non-linerarities to be compensated for.
In the article "Linear Transceiver Architectures", 1988 IEEE Vehicular Technology Conference, pp. 478-484, A. Bateman, et al, disclose how power amplifiers having high linearity can be obtained alternatively with the aid of cartesian feedback, the LINC principle all adaptive pre-distortion (LINC=Linear amplification with Non-linear Components).
In the article "Linear Modulators Based on RF Synthesis: Realization and Analysis", IEEE Transactions on Circuits and Systems--I: Fundamental Theory and Applications, Vol. 42, No. 6, July 1995, K.-Y. Chan, et al, analyse different methods of creating QPSK-modulated radio signals by radio frequency synthesis (QPSK=Quadrature Phase Shift Keying). All methods involved utilise the so-called CALLUM principle, which is a further development of the LINC principle (CALLUM=Combined Analogue Locked Loop Universal Modulator).
V. F. Dias, et al, render in their article "Sigma-Delta Modulators for High-Resolution and Wide-Band A/D Converter Applications", Microelectronics Journal, 25(1994, pp. 253-277, an account of broadband applications for single-bit, multi-bit and cascade coupled sigma-delta modulators.
It is known from Patent Specification U.S. Pat. No. 5,401,953 to utilise sigma-delta modulators in single and exponential radio frequency modulation, i.e. radio frequency modulation of quadrature-divided signal components. European Patent EP-B1 426 560 also discloses a solution in which sigma-delta modulators are used to modulate an input signal exponentially.
In the article "Complex-Signal Sigma-Delta Modulators for Quadrature Bandpass A/D Conversion", Microelectronics Journal, Vol. 27, No. 6, 1996, pp. 505-524, V. da Fonte Dias gives an account of the advantageous noise filtering that can be achieved with the use of complex sigma-delta modulators. These modulators make possible noise transmission functions that are not symmetrical with respect to d.c. voltages. As a result, the design methodology can be made very simple with respect to lower and higher order cascade topologies for this type of modulator.
The article "Progress in UWB Generation with Linear Silicon Switches", Optically Activated Switching III, SPIE, Vol. 1873, 1993, pp. 238-248, by K. Cardwell, et al, disclose methods of generating ultra bandwidth radar pulses with the aid of light-activated silicon switches, LASS (LASS=Light Activated Silicon Switch).
Two different methods of providing very high frequency electromagnetic pulses with the use of a pulse laser, optical delay device and a photoconductive switch are disclosed in the article "High Speed, High Resolution Analogue-to-Digital Conversion using a Hybrid Electro-Optic Approach", 1995 IEEE International Symposium on Circuits and Systems, pp. 704-707, by R. Mason, et al, and in U.S. patent specification U.S. Pat. No. 5,401,953.
Highly reliable, long lifetime silicon photoconductive switches are known from the article "Long Lifetime Silicon Photoconductive Semiconductor Switches", Optically Activated Switching III, SPIE, Vol. 1873, 1993, pp. 27-38, by A. Rosen.
The production of plasma for fusion reactors, the detection of X-ray radiation and the generation of broadband high power pulses in pulse radar applications are examples of further fields in which optically activated switches are used.